Zircon was born as a branch of LK and even now many inner constructs are based on LK while the layers above are new. For example, Zircon has the concept of a process but LK does not. However, a Zircon process is made of LK-level constructs such as LK's
LK is a Kernel designed for small systems typically used in embedded applications. It is a good alternative to commercial offerings like FreeRTOS or ThreadX. Such systems often have a very limited amount of ram, a fixed set of peripherals and a bounded set of tasks.
On the other hand, Zircon targets modern phones and modern personal computers with fast processors, non-trivial amounts of ram with arbitrary peripherals doing open ended computation.
More specifically, some the visible differences are:
Over time, even the low level constructs have changed to accomodate the new requirements and to better fit the rest of the system.